Regarding the change to the nikud of nachalot to nechalot. According to a book called "Luchot Hasheimot Hashaleim" by Dr. Shaul Barkali, it would appear that the assumption that the plural of "nachlah" is "nachalot" is wrong, as in fact the plural of nachalah is "nechalot". (As an aside, the word "nechalot" also appears in Joshuah 19:51 whereas the word "nachalot" does not appear anywhere in Tanach.)
According to Dr. Barkai, whenever you have a word that ends in a kamatz followed by a hey (like nachlah) if the syllable before the kametz is closed by a shva nach, then in plural without semichut the first letter becomes a shva and the second letter a kamatz. Thus nachalah, becomes nechalot and simlah becomes semalot (see Exodus 12:34). (If the first letter of the work is a gutteral i.e. aleph, hey, chet, ayin, the first letter becomes a chataf patach, however the second letter still becomes a kamatz and as such agalah, becomes agalot with a kamatz (see Genesis 45:21)). In semichut or netiah (nachalati etc) however, the kamatz goes away and the original nikud of the first letter remains. Thus, the switch to a kamatz under the chet is going to be very rare, as the rule applies in a very limited circumstance, i.e. kametz hey ending, preceded by shva nach, in plural without semichut or netiyah.
The reason for this change of nikud is simply to make the word easier to pronounce.
Tzvi Daum
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